A Stirling refrigeration unit has been known as a compact refrigeration unit operable at fairly low temperatures with a high performance coefficient and a high refrigeration efficiency. In addition, it has an advantage that it is operable with an environmentally friendly Freon substitute.
For this reason, a Stirling cooling unit has been considered as a versatile refrigeration unit for use in, for example, refrigerators, freezers, throw-in type air conditioners for business and domestic use, and low-temperature fluid circulation systems, constant low-temperature boxes, thermostatic ovens, heat-shock test equipments, freeze-drying machines, temperature-characteristics test equipments, blood and cell preservation apparatuses, coolers, and various kinds of measuring apparatuses.
FIG. 1 is a schematic circuit diagram of a Stirling refrigeration system 100. As shown in FIG. 1, the Stirling refrigeration system 100 comprises Stirling refrigeration unit 110, a water-cooling unit 120, and a heat transport unit 130. Cold-refrigerant is provided to a refrigeration apparatus 101.
In the example shown in FIG. 1, in order to attain high refrigeration performance, the Stirling refrigeration system 100 is provided with two Stirling refrigeration units 110 connected in series, along with two water-cooling units 120.
Each of the Stirling refrigeration units 110 comprises a compression unit 114, an expansion unit 118, and a heat accumulation unit 119, as shown in FIG. 2. The compression unit 114 has a compression cylinder 112 and a compression piston 111 for compressing a working gas contained in the compression space 113 defined by the compression cylinder 112 and the piston 111. The working gas is compressed by the piston 111. The expansion unit 118 allows the working gas in an expansion space 117 defined by an expansion piston 115 and an expansion cylinder 116 to expand as the expansion piston 115 reciprocates in the expansion cylinder 116. A heat accumulation unit 119 is provided in a gas passage S communicating with the compression space 113 and the expansion space 117.
A motor 109 is provided to drive a crank mechanism 108 for converting the rotational motion of the motor 109 to reciprocal motions of the compression piston 111 and the expansion piston 115, thereby compressing and expanding the working gas.
The compressed working gas passes through the gas passage S to the heat accumulation unit 119, where the gas is cooled, and then to the expansion space 117 where the gas expands to lower its temperature.
A cold head 131 provided in the header of the expansion unit 118 is cooled by the cold gas. Thus, a secondary refrigerant circulating through the cold head 131 is cooled.
After expanded in the expansion unit 118, the working gas returns to the compression unit 114 through the heat accumulation unit 119, completing the Stirling cycle.
It is noted that the compression piston 111 leads the expansion piston 115 in phase by about 90 degrees.
The heat transport unit 130 shown in FIG. 1 has a secondary refrigerant pump 132, a tank 133, a liquid-gas separator 134, and a pressure regulation bellows 135. The secondary refrigerant pump 132 pressurizes the secondary refrigerant, causing the secondary refrigerant to circulate through a secondary refrigerant circuit connecting the cold head 131 with the refrigeration apparatus 101. The tank 133 adjusts the flow of the secondary refrigerant that circulates through the secondary refrigerant circuit. The liquid-gas separator 134 receives the secondary refrigerant returning from the refrigeration apparatus 101 and separates the liquid component and the gaseous component of the refrigerant (the separation will be referred to gas-liquid separation). The pressure regulation bellows 135 absorbs pressure fluctuations occurring in the secondary refrigerant circuit.
The liquid-gas separator 134 has a gas-liquid separation tube 136 and a gas recovery tube 137. The gas-liquid separation tube 136 has a shape of a generally inverted U-shape and is connected to a tube for returning the secondary refrigerant from the refrigeration apparatus 101. The gas recovery tube 137 has one end connected to a top section of the gas-liquid separation tube 136, and another end connected to the upper end of the tank 133 to communicate with the free space of the tank 133.
The gas-liquid separation is performed in the gas-liquid separation tube 136 by causing only the gaseous component of the refrigerant to flow upward in the tube 136 when the secondary refrigerant flow upward in the gas-liquid separation tube 136 after it has returned from the refrigeration apparatus 101. The gaseous secondary refrigerant separated in the gas-liquid separation process is stored in the tank 133 via the gas recovery tube 137.
It will be understood that the secondary refrigerant undergoes a volumetric change due to a change in temperature as it refrigerates the refrigeration apparatus 101.
Since the heat transport unit 130 is a closed cycle, the pressure in the secondary refrigerant circuit will change if the secondary refrigerant changes its volume. The pressure regulation bellows 135 alleviates this pressure change.
Thus, the pressure regulation bellows 135 is adapted to increase its length as the pressure in the secondary refrigerant circuit increases, and decreases its length as the pressure decreases. Accordingly, the pressure inside the secondary refrigerant circuit is maintained at a substantially constant pressure.
It should be noted that the temperature of the working gas rises as it is compressed in the compression unit 114 and that the efficiency of cooling the working gas would undesirably decline if the hot working gas were directly led to the heat accumulation unit 119 before it is led to the expansion unit 118.
Hence, in order to circumvent this adverse effect, the working gas is cooled by the water-cooling unit 120 provided in the gas passage S between the compression unit 114 and the heat accumulation unit 119.
The water-cooling unit 120 includes a heat exchanger (not shown)for cooling the working gas (referred to as working gas end (WGE) heat exchanger), a radiator 121, and a cooling-water pump 122. The WGE heat exchanger effects heat exchange between the working gas and the cooling water. The radiator 121 effects heat exchange between the cooling water and the atmosphere. The cooling-water pump 122 circulates the cooling water through the WGE heat exchanger and the radiator 121.
The radiator 121 has a main body which includes a continuous fluid tube 125 having a multiplicity of parallel sections 123 and curved sections 124 connecting the parallel sections, and a plurality of fins 126 fitted on the parallel sections 123.
Fins 126 effect heat transfer between the cooling water flowing inside the fluid tube 125 and the atmosphere.
In this way the working gas is cooled by liberating its thermal energy from fins 126 to the atmosphere to enhance refrigeration efficiency.
As described above, this Stirling refrigeration system 100 has two Stirling refrigeration units 110 and two water-cooling units 120, to enhance its refrigeration power. For this reason, Stirling refrigeration system 100 described above also requires two cooling-water pumps 122. This arrangement, however, has a disadvantage that Stirling refrigeration system becomes large in dimensions and hence requires a large installation area.
Further, deaeration of the cooling-water circuit is difficult, though it is necessary when filling cooling water to the water-cooling unit 120. This can happen because the water-cooling unit 120 includes many vertical tubes and curved sections (convex sections) connecting the vertical tubes, which can easily trap air. Hence, a considerable amount of air remains in the tubes.
If the air continues to remain in the tubes, the cooling-water pump 122 will fail pumping the refrigerant, only “biting” the air (the failure of pumping referred to as “air-biting”), which can damage the shaft of the pump 122 and cause a loud noise.
Furthermore, since the fluid tube 125 of the radiator 121 has many parallel section 123 and curved sections 124, if the parallel sections 123 are inclined during the installation of the radiator 121, air will remain inside the parallel sections 123, which will degrade the heat radiation efficiency of the water-cooling radiator 121.